Exposure to flaxseed and its purified lignan reduces bone strength in young but not older male rats

Flaxseed is the richest source or the plant lignan secoisolariciresinol dig lycoside (SDG), which is converted to the two major mammalian lignans, ente rodiol (ED) and enterolactone (EL), by colonic bacteria. Because both ED an d EL can produce biological effects similar to estrogen, exposure to lignan s during early stages of development may adversely alter the normal develop ment of bone in males since bone is a hormone-sensitive tissue. To determin e whether early exposure to flaxseed or its lignan compromised the acquisit ion of bone mass or reduced hone strength, male offspring were exposed to o ne or three diets during lactation only (birth through postnatal day (PNDI 21) via mother's milk or continuously from the start of lactation through t o adolescence (PND 50) or young adulthood (PND 132). The diets were a basal diet (BD) that was devoid of phytoestrogens, BD containing 10% flaxseed, o r BD containing the equivalent quantity of SDG present in a 10% flaxseed di et. To assess hone quantity, the bone mineral content (BMC) and bone minera l density (BMD) of femurs were assessed by dual-energy x-ray absorptiometry . Since the biomechanical properties of bone are indicators of the microarc hitecture architecture and thus bone quality, the biomechanical strength of femors was assessed by three-point bending. At PND 50, ultimate bending st ress and Young's modulus, measures of hone strength, were reduced among rat s that received the 10% flaxseed diet from PND 0 through PND 50, while ther e were no marked differences in bone size, BMC, or BMD among groups. Intere stingly, this effect does not appear to be due to the lignan in flaxseed, a s continuous exposure to the diet containing the equivalent quantity of lig nan (10 S diet) did not alter any measures of bone strength, in contrast to PND 50, bone strength did not differ among groups at PND 132, indicating t hat the compromise in bone strength was not sustained into early adulthood. Bone size, BMC, and BMD continued to he similar among treatment groups at PND 132. In conclusion, exposing male rats to a diet containing 10% flaxsee d or an equivalent quantity of lignan either during lactation only or throu gh to early adulthood is safe with respect to bone health, as measures of b one mass and strength were similar to control rats.